Older Toro 580 D trouble

I have an older 580 D Toro that I have had 3 different mechanics out working on an after spending $2,000 dollars still have the same problem. When I mow taller grass the front and right hand deck do great but the left hand mower just lays the grass over. It almost acts they the blades loose the speed and power to cut it. They keep messing with the hydraulic panel on the left front side of the machine. I am told pressure checks all come out right and that seals have all been replaced. The motor is not leaking and I find no leaks on the lines going to that drive motor. Is there a way to test that drive motor to see if it is loosing power under a load? Should I just replace the drive motor? Has anyone else had a problem similar to this?
On normal height grass this mower runs like a champ. I love the mower just cant find why the problem. Please email me at ddowney66040@outlook.com with any suggestions.